Shatapatha Brahmana 1.7.1.20

1.7.1.20

athodakavatottānena pātreṇāpidadhāti | nedenadupariṣṭānnāṣṭrā rakṣāṃsyavamṛśāniti vajro vā āpastadvajreṇaivaitannāṣṭrā rakṣāṃsyato 'pahanti tasmādudakavatottānena pātreṇāpidadhāti

He then covers it over by a vessel, with the hollow part upwards and containing water, 'lest the evil spirits, the Rakshas, should touch it from above;' for water, indeed, is a thunderbolt; hence he thus drives away from it the evil spirits, the Rakshas, with a thunderbolt: this is the reason why he covers it over by a vessel with the hollow part upwards and containing water.